Field
The present invention relates to synchronizing, logging and post processing metrology instrument readings with the position and orientation of precision production equipment such as: computer numerically controlled (CNC) mills and lathes, grinding machines, lapping machines, robots, additive manufacturing equipment (a partial list of precision production equipment). More particularly, as an example, the invention relates to synchronizing non-contact dimensional metrology instruments with precision production equipment in order to characterize a workpiece's shape and features while it is loaded in the work space of precision production equipment.
Related Art
At the time of writing, a computer numerically controlled (CNC) machine is capable of manufacturing a workpiece or product to within 2 micrometers (10−4 inches) of design specifications under close to ideal conditions. Some precision production equipment can achieve even tighter dimensional tolerances. However, it is difficult to produce CNC machine tools that are capable of high fidelity measurements of said workpiece with absolute accuracy on the order of micrometers (10−4 inches). Further, machining and measurement are typically done in separate operations with specialized equipment dedicated to each operation.
Modern CNC machines typically use a drawing or digital model from which is created a profile for the product and/or instructions. The instructions or profile are used to control the operation of the CNC machine. A programmer or operator may be involved to design a manufacturing process from the drawing or model. A numerically controlling program is created manually or automatically through an automated programming device. An operator enters or selects an appropriate numerically controlling program and manually sets a starting material for the workpiece in the CNC machine. Alternatively, the starting material is automatically placed therein.
Subsequently, the CNC machine creates a product by following the set of instructions. The CNC machine cuts, grinds, drills and shapes a workpiece from the starting material. Before processing or manufacturing begins, setup requires many steps including establishing a workpiece coordinate system prior to machining and establishing the maximum material condition such that the first machining pass for each feature removes minimal material (and ensures each tool does not crash into the workpiece). Such setup can be tedious and time-consuming for large geometrically complex parts such as castings and weldments.
Further, just after the product or workpiece is created, it is unknown whether the particular workpiece matches in all respects the drawing or digital model. Conventionally, one way to determine the dimensions, shape and size of a finished workpiece is to use a touch probe and have a coordinate measuring machine (CMM) or CNC machine utilize the touch probe to contact and pause (stop all machine motions) at discrete points of each workpiece feature of interest.
The common touch probe technology measurement method routinely involves four distinct phases for each discrete point. During the first phase the probe is maneuvered along a safe path to a point in space that is along a normal vector from the surface feature of interest. The second phase involves maneuvering the probe along the normal vector until contact with the feature is detected by the probe (mechanism of contact detection internal to the probe body can be a set of contacts, strain gage(s), or optically). After contact is detected, machine motion stops; this pause at a single position enables the precise capture of all spatial variables of the machine and probe. Then an offset is applied to these spatial variables to compensate for the probe tip diameter and the approach vector, thereby computing a discrete point in space corresponding to the feature of interest. The coordinates of this point are stored in memory. The fourth phase generally involves a retreat along the original normal vector to a safe point to start the first phase of the next probe point measurement.
Such procedure is fraught with drawbacks. For example, taking such measurements over the surface of most shapes and workpieces is time-consuming. Further, the stylus ball at the end of the touch probe inherently limits the minimum feature size tactile feedback is capable of, certainly compared to other means of measurement (e.g., laser sensors, optical sensors). Only a limited number of measurements are realistically possible with discrete tactile measurements as determined by the time budget for inspection and the average time between touch probe operations. When the common practice of part verification is conducted in a dedicated instrument, such as a CMM, separate from the production equipment, such as a CNC milling center, the part coordinate system must be established in each operation. The variability involved with establishing the part coordinate system multiple times in multiple machines creates a source of error if and when the same component requires rework in the production equipment. When rework is required, the accuracy requirements of the subsequent set up in the production equipment are increased significantly and may require even more care and time to achieve the part coordinate system and maximum material condition.
Some scientists and engineers have attempted continuous tactile and non-tactile means of performing measurements of workpieces in production equipment. As mentioned previously, the common state of the art for measurement in production equipment involves gathering discrete points, each point requiring on the order of one second, in many instances as much as two seconds per point.
Near continuous measurement tools with rates of thousands of points per second that do not require the production equipment to physically stall at each point has long been desired by industry. However, synchronizing such measurements with the positions and motions of precision production equipment such as a CNC machine has been problematic. Most metrology instruments process their input signal and thus impose a slight temporal delay in reporting their measurements. The metrology instrument temporal delay may be on the order of 400 micro-seconds. This instrument time delay is not an issue when the instrument is utilized to take discrete points with the method outlined above. However, the delay is an issue while attempting to characterize a workpiece with a continuous scan that does not pause in space momentarily to log its readings.
FIG. 1 shows a two-dimensional schematic of a CNC machine according to a conventional use. With reference to FIG. 1, a milling machine 100 includes a headstock 102 from which the milling machine 100 controls a spindle 106 and operating arm 108. Attached to the operating arm 108 is a tool head 110 and touch probe 112. The operating arm 108 brings into contact with a workpiece 116 the end 114 of the touch probe 112. The milling machine 100 knows the location of the end 114 of the touch probe and records a set of positional values when the touch probe 112 detects a mechanical resistance. The workpiece 116 is clamped or otherwise fixed on a movable bed 118. The positions of the various parts of the milling machine 100 (e.g., operating arm 108, bed 118) are monitored and recorded. The bed 118 rests on a rigid frame 120, and the column 122 houses various mechanical, electric and computer-based components. The milling machine 100 may be, for example, a five-axis CNC machine where the axes include: an x-axis, a y-axis, a z-axis through which the operating arm 108 and tool head 110 may be operated; and two axes of rotation (e.g., a-axis, and b-axis) along which the workpiece 116 may be rotated or moved. Commonly the touch probe 112 is removed during cutting operations and installed directly into the spindle 106 during measurement operations without requiring the operating arm 108.
As known in the art, and as can be inferred from FIG. 1, measurements related to the position of the end 114 of the touch probe 112 are taken while the production equipment is paused and the contact probe is engaged with the workpiece. For complex shapes, it is excessively time-consuming to get a sufficiently accurate set of measurements from which to build a model of the particular workpiece 116 in the milling machine 100. While this practice is adequate for establishing workpiece coordinate systems, it is deficient in characterizing the maximum material condition of complex weldments and castings, or verification of workpiece features. Due to the sparse characterization of maximum material condition the operating instructions start out in free space and approach the workpiece in a seemingly timid or cautious manner so as to avoid a crash into the workpiece 116. Once the machining process is complete, the tactile probe may be used to gather a few points associated with each key feature before removing this workpiece 116 and loading the next. Thus, conventional uses include spot-checking a few key sizes or locations of a workpiece 116 before a new blank or starting block is placed in the milling machine 100. Some engineers and operators have attempted to create a continuous or near continuous (hundreds or thousands of points per second) production machine-based measurement system by replacing the touch probe 112 with electronic sensors (not shown). However, there remain substantial shortcomings of repeatedly moving the operating arm 108 to a new location and taking a single measurement with the sensor (not shown), and making a single recording of the position of the sensor related to the workpiece 116 and then synchronizing these two measurements such that in the physical domain they are mechanically aligned to at least the production equipment manufacturer's stated position accuracy and repeatability. These and other disadvantages are overcome with the teachings described in the present invention.